This invention relates generally to injection molding apparatus for particulate synthetic resinous material which has a tendency to score internal surfaces of the apparatus. More specifically, the invention concerns a screw design which reduces the torque requirements imposed on the screw drive and which substantially eliminates scoring of cooperating surfaces of the barrel and the screw of an injection molding machine.
Rotary screws have long been used in injection molding apparatus to accept synthetic resinous material in solid particulate form and plasticize the material into a viscous fluid for subsequent injection into hollow molds. In the past, some synthetic resinous materials, such as polyurethane, have been known to impose high torque requirements on the screw drive and to exhibit an undesirable tendency to score and abrade cooperating interior surfaces of processing machinery. Surfaces which are typically damaged include the bore of the housing, the rotary screw and the flights of the rotary screw. The provision of high torque capacity is expensive and represents a generally undesirable approach to adapting molding machines for the processing of polyurethane and other tough resins.
Surface scoring eventually requires replacement or reworking of the affected parts with the appurtenant expense and loss of production time. Since injection molding apparatus is conventionally used in commercial operations, such expense and time loss become economically significant and minimization thereof becomes highly desirable.
Overfeeding of particulate material caused by the design of the rotary screw is a significant factor in the scoring problem. Where overfeeding does occur, individual particles of feed stock material are caught and wedged between flights of the screw and the bore of the housing. In addition to screw design, overfeeding may result from the inability of downstream portions of the screw to physically accept more material as a result of existing back pressure in plasticized material.
The prior art discloses apparatus having a small carefully controlled clearance which insignificantly affects operation of the screw and, therefore does not address the overfeeding problem. In addition, preferentially abradable materials are disclosed for fabrication of screw portions thus increasing the number of parts required, the expense of the apparatus, and the frequency of repairs.
Other concepts such as the use of specialized materials for the barrel or screw or specialized surface treatment to inhibit scoring and abrasion have also been employed in the past. These, however, do not deal with the causes of the scoring.
Thus, the previously known attempted solutions to the problem of scoring and abrasion have not been entirely satisfactory or effective. Moreover, since scoring and abrasion commonly results simply from overfeeding particulate material, failure to address and solve the cause of scoring severely limits the utility of previously proposed solutions.
Accordingly, it would be desirable to have a screw for use in injection molding apparatus which not only reduces the torque requirements imposed on the screw drive, but also substantially reduces scoring and abrasion of internal surfaces while simultaneously minimizing the most common cause thereof.